1. Field of the Invention
The invention relates to the field of communicating digital data signals over lines such as twisted pair lines.
2. Prior Art
There is an enormous installed base in the United States and elsewhere of twisted pair lines and similar lines. For the most part, these lines were installed to carry voice communications confined to lower frequencies. The recent need for additional bandwidth, for example to connect home and businesses to the Internet, has presented both opportunity and challenges to better utilize this installed base for higher speed communications.
Integrated circuits are available which provide duplex communications with echo canceling capability and adaptive equalization. These circuits are designed to operate at a selected speed which is selected based on criteria such as the length of a line and the gauge of wire in the line. For instance, for a twisted pair line of 19.0 k feet with #24AWG wire, a data rate of 528 kbps may be selected. On the other hand, if a distance is increased for the same wire to 20 k feet, the data rate may be reduced to 400 kbps.
Selection of data rates based on line length and other physical characteristics does not necessarily optimize the bandwidth of the line since there are numerous other variables which affect the frequency response of the line such as the condition of the line, its environment, interference, etc.
As will be seen, the present invention adds another layer of adaptation to more fully realize the bandwidth capabilities on a digital data line particularly a dedicated line.
An apparatus and method for communicating over a data signal line is described. In one embodiment, the apparatus includes an interface circuit having an analog circuit and a digital signal processor (DSP) which are coupled to received and transmit data onto the line. The apparatus includes a controller coupled to the interface circuit. The controller provides timing signals at, at least, two different frequencies selected under the control of a first control signal. An oscillator in the controller receives a second control signal from the interface circuit used by the oscillator in a receive phase for controlling the timing signals. The controller includes a state machine having states corresponding to the two different frequencies of the timing signals. This state machine provides the first control signal.
In one method of the present invention, initiating of communications is attempted at a first data rate or speed corresponding to a first frequency. If communications are not successful at the first speed, reactivation is attempted at a second speed corresponding to the second frequency, lower than the first speed. On the other hand, if communications are successful at the first speed, then communications are continued at the first speed. If communications are successful for a predetermined period at the first speed and the line then appears to be bad, there is an attempt to reactivate at the first speed rather than reactivating at the second lower speed. If this reactivation at the first speed is unsuccessful then reactivation at the lower speed is attempted.